Thermoelectric generation relies on a fuel source (fossil, nuclear, or biomass) to heat water to steam in a boiler that drives a turbine that then turns a generator creating electricity. Generally there are a series of turbines utilizing the steam energy. The remaining steam, having energy too low to drive another turbine, must be cooled to condense back to water where it is recycled back to the boiler to repeat the process. The condensation of the steam is accomplished in tube and shell heat exchangers also known as condensers. The steam is condensed on the shell side by the flow of cooling water through tubes bundled inside the condenser. As the steam cools, the latent energy is transferred to the cooling water which is circulated to cooling towers. The heated water is then allowed to cascade downward on an open air medium where fresh dry air is passed across the medium. The dry air becomes more humid and evaporates, thereby removing the residual heat from the system.
This condensing process results in huge water and energy losses. Approximately sixty percent of the energy in a coal fired power plant originally developed at the boiler is wasted in the form of evaporation in the process of condensing low grade steam. Power plants are always attempting to lessen these losses.
The National Energy Technology Lab (NETL) completed a study in 2007 based on a normalized comparison of seven fossil fuel power plant designs. For each plant, heat and material balances were prepared on a common basis with emphasis on Water Usage and Water Loss, where Water Usage was the raw metered water from external sources and used in the power plant for any and all purposes and Water Loss was the water exiting the system and that represents the overall “loss” of water to the environment.
According to this basis, Water Loss is greater than Water Usage because water enters the system in the fuel and from ambient air (combustion processes liberate water). Eighty to ninety-nine percent of the power plant raw water usage was through a combination of cooling tower evaporation and blowdown.
It is desirable to decrease water consumption in power plants, particularly in light of ever increasing demand on limited fresh water. As populations grow so does the need for energy and fresh water. A problem is that creating energy requires large amounts of fresh water, so even incremental improvements in lowering the water requirements for energy production are needed.
Accordingly, what is needed is a way to reduce water requirements in steam generation and other industrial processes in a more energy efficient manner.